net/sff8472.h: add double-include protection

Reviewed by:	emaste
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D50051

sys/net/sff8472.h

@@ -26,6 +26,9 @@
  * SUCH DAMAGE.
  */
 
+#ifndef _NET_SFF8472_H_
+#define _NET_SFF8472_H_
+
 /*
  * The following set of constants are from Document SFF-8472
  * "Diagnostic Monitoring Interface for Optical Transceivers" revision
@@ -526,3 +529,5 @@ static const char *sff_8024_id[SFF_8024_ID_LAST + 1] = {
  */
 
 #define SFF_8472_POWER_FACTOR 10000.0
+
+#endif